Hydrogen has been used in large quantities as a basic material gas in petrochemistry. In particular, hydrogen has recently received attention as a clean energy source in fields, such as fuel cells. Thus, hydrogen is expected to find wider use. Hydrogen for use in such applications has been produced by reforming of water vapor or carbon dioxide, a partial oxidation reaction, or a decomposition reaction, from raw materials mainly composed of hydrocarbons, such as methane, butane, and kerosene, and oxygen-containing hydrocarbons (hydrocarbons containing an oxygen atom), such as methanol, ethanol, and dimethyl ether, followed by separation with a permselective membrane that is selectively permeable to hydrogen, such as a palladium alloy film. Among the raw materials for hydrogen production, ethanol can be produced from biomass and is therefore a promising next-generation carbon-neutral raw material.
In recent years, hydrogen has been produced using a permselective membrane reactor (membrane reactor), in which the reaction and the separation as described above can simultaneously be performed (see, for example, Patent Document 1). Widely used permselective membrane reactors include a reactor tube that has a gas inlet at one end and a gas outlet at the other end, a porous separator tube that is disposed in the reactor and has a permselective membrane selectively permeable to hydrogen on the surface, and a catalyst that promotes the reforming of a hydrocarbon and/or an oxygen-containing hydrocarbon.
In general, the reforming catalyst has a pellet shape, and is placed between the reactor tube and the separator tube, or is packed in the separator membrane (packed bed). A raw material gas supplied from the inlet to the reactor comes into contact with the reforming catalyst and is decomposed into hydrogen and other gases, for example, by steam reforming. For example, in steam reforming of methane, the reforming catalyst promotes a reforming reaction expressed by the following reaction formula (1) and a shift reaction expressed by the following reaction formula (2). Thus, a hydrocarbon (methane) is decomposed into reaction products, such as hydrogen, carbon monoxide, and carbon dioxide, producing a gas mixture (gaseous product) containing the reaction products.CH4+H2O→CO+3H2  (1)CO+H2O→CO2+H2  (2)
Hydrogen in the gaseous product passes selectively through the permselective membrane into the separator tube, and is thereby separated from other gas components and recovered. Other gas components that do not pass through the permselective membrane, such as carbon monoxide and carbon dioxide, are discharged from the reactor through the gas outlet of the reactor tube.
The permselective membrane reactors can simultaneously perform the catalytic chemical reaction and the hydrogen separation with a permselective membrane. This advantageously simplifies the structure of an apparatus and reduces the footprint of the apparatus. In addition, the elimination of a hydrogen product from the reaction system through the permselective membrane shifts the equilibrium of the chemical reaction toward the product, thus allowing for a lower temperature reaction. A lower temperature reaction consumes less energy during the reaction and prevents the reactor material from deteriorating. More specifically, while the reaction temperature is in the range of about 600° C. to 800° C. in conventional non-membrane reactors, which have no permselective membrane, the reaction temperature is in the range of about 400° C. to 600° C. in permselective membrane reactors.
However, in the hydrogen production using the permselective membrane reactors, although the reaction temperature is advantageously reduced, a disproportionation reaction of carbon monoxide expressed by the following reaction formula (3) occurs more frequently, causing deactivation of a catalyst due to coking.2CO→C+CO2  (3)
The catalyst deactivation due to coking also occurs in the conventional non-membrane reactors. However, while the main cause of coking is a decomposition reaction of a hydrocarbon in the non-membrane reactors, it is the disproportionation of carbon monoxide in the permselective membrane reactors as described above. In the hydrogen production using the permselective membrane reactors, therefore, the catalyst deactivation due to coking must be prevented by a particular measure different from that in the non-membrane reactors.
Furthermore, because hydrogen produced by a catalytic reaction diffuses through space of a packed catalyst layer, hydrogen cannot move smoothly to the permselective membrane. This reduces the efficiency of separation and recovery. Such a problem is particularly significant in permselective membranes having high permeability.
Patent Document 1: Japanese Unexamined Patent Application Publication No. H06-40703